Lipotoxic diseases refer to a group of organ/tissue disorders caused by lipid overload (steatosis) in the tissues or organs involved. They are commonly seen as complications of obesity, lipodystrophy, and the metabolic syndrome. Steatosis-induced muscle insulin resistance and metabolic cardiomyopathy, for example, are manifestations of lipotoxic diseases. In order to understand the underlying mechanism of lipotoxicity, and in particular, to identify causative lipid derivative(s) that accumulate during the development of steatosis, we propose to develop two transgenic mice with tissue-specific overexpression of the Dgat1 (diacylglycerol acyltransferase-1, or DGAT1) gene. In these mouse models, fat (triglyceride) content of the cells in which Dgat1 is overexpressed is expected to increase, because this gene encodes a key enzyme involved in making triglyceride. However, this increase in fat content might be an isolated event, without necessarily being accompanied by an increase in the uptake of lipid fuels by the cells. In this regard, these mouse models would differ significantly from other existing mouse models, in which genetic alterations lead to increased uptake of lipid for fuel consumption, with "spillover" of those fuels into triglycerides. The latter mouse models are associated with tissue steatosis and lipotoxic diseases. With our mouse models, we will be able to determine if isolated increases in triglyceride content in affected muscle cells will lead to lipotoxic muscle dysfunction. Hence, we will be able to determine if increased deposition of fat in the cell is by itself toxic. Because increases in DGAT1 activity will help shunt lipid metabolites away from pathways that produce potentially toxic lipid derivatives, these mouse models will also be useful to test the hypothesis that increased production of DGAT1 will prevent, or actually reduce the risk of, lipotoxicity when cells are overloaded with lipid fuels. In summary, the results from these proposed studies will provide important information about the actual mechanistic links between lipid metabolism and lipotoxic diseases. As a result, we may be able to identify new molecular target(s) for future drug therapy aimed at reducing lipotoxicity